Generation of Near-Primary Mantle Melts

A spectrum of near-primary mantle melts are injected into the crustal plumbing systems of volcanoes at convergent plate margins, which are attributed to a combination of anhydrous decompression melting, hydrous flux melting, and crystallization in the mantle wedge. However, the exact mechanisms and spatial distribution of these processes in the upper mantle remain enigmatic.

1. Hydrous Arc Magma Genesis at Convergent Plate Margins

My research uses high-pressure piston-cylinder and multi-anvil experiments to investigate the conditions required to generate the first melts of water-rich mantle near the downgoing plate-mantle wedge interface in subduction zones (Till et al., Cont. Min. Pet., 2012a,b,c). Combining these observations with geodynamic models has provided insight into the location of arc volcanism with respect to the trench (Grove et al., Nature, 2009). Our experimental results also show that the mineral chlorite is an important carrier of H2O to the locations of hydrous melting in the mantle wedge at subduction zones. Field investigations of the Higashi-Akaishi peridotite in Japan and the Cima di Gagnone peridotite in Switzerland suggest chlorite can be stable in the mantle wedge, whereas previous studies focus on its presence in the downgoing plate.

A related problem is unraveling the spatial distribution of anhydrous melting both in the mantle wedge at arcs and at other continental tectonic settings. Using our melting model for plagioclase, spinel (Till et al., JGR, 2012) and garnet (Grove et al., CMP, 2013) lherzolite, it is possible to calculate the pressure and temperature of origin for near primary basaltic magmas or forward model batch or near-fractional melting of nominally anhydrous mantle of a known composition.

The model was calibrated for application to a wider range of mantle bulk compositions than predecessor models such as the model for MORB genesis by Kinzler & Grove, 1992 and Kinzler, 1997. As such it is particularly well-suited to examining melt formation in continental settings. In addition, the model was calibrated with only experiments that contain all the mantle mineral phases (olivine, clinopyroxene, orthopyroxene and the relevant aluminous phase) in addition to melt, and therefore the model is best suited to modeling relatively low degrees of mantle melting (<20-25%). As an example, the model has been applied to Quaternary basalts erupted in the southern Cascades arc and back-arc (Till et al., G-cubed, 2013; Long et al., G-cubed, 2012).

Application of this lherzolite melting model to primitive basalts from a variety of tectonic settings can also provide independent estimates of the depths of the lithosphere-asthenosphere boundary, as well as the origin of its observed seismic properties (Till et al., G-cubed, 2010).